There are a few other potential causes, for example:
– the LCD might be damaged, although this is very unlikely unless if you have reason to believe that it’s damaged by 24VAC
– the MCU’s reset line (pin 1) is for some reason held to ground, causing the MCU to remain in reset state (also very unlikely since you can program the MCU successfully).
I am applying a DC 12V using bench power supply and before I put in any chips, I measured 5V and 3.3V like in your tutorial. LCD does not seem to be dead. Pin 1 of MCU is pulled high to 3.3V.
At this point, i don’t know what could be wrong. Do you have a program I can flash to ensure the mcu is running? Usually i have a serial terminal that I can see if things are alive or not.